Árbitro vs. juiz
In Portuguese, árbitro and juiz can both translate to referee or umpire, but their usage depends heavily on the country and the level of formality. While árbitro is the technical and universal term, juiz is frequently used colloquially in Brazil.
Árbitro
B1This is the official, formal term for a referee or umpire used in professional media, rulebooks, and official announcements in both Brazil and Portugal. In Portugal, this is the standard word used in almost all contexts.
O árbitro apitou o início da partida pontualmente.
(The referee blew the whistle for the start of the match on time.)
Ele estudou muito para se tornar um árbitro da FIFA.
(He studied hard to become a FIFA referee.)
O árbitro de vídeo anulou o gol por impedimento.
(The video referee annulled the goal due to offside.)
Juiz
B1Literally meaning judge, this word is commonly used in Brazil as a colloquial synonym for a referee, especially in football (soccer). In Portugal, juiz is rarely used for field sports and is usually reserved for officials who award points in performance sports like gymnastics or surfing.
A torcida ficou brava e xingou o juiz o jogo inteiro.
(The fans got angry and cursed at the referee the whole game.)
Ninguém concordou com a decisão do juiz naquele lance.
(No one agreed with the referee's decision in that play.)
O juiz deu cartão vermelho para o zagueiro.
(The referee gave a red card to the defender.)
Summary
To sound correct in any situation, árbitro is the safest choice as it is the official term in both countries. However, you will hear juiz very often in informal conversations among Brazilians watching sports, whereas in Portugal, calling a football referee a juiz sounds unnatural.